Optically anisotropic carbonaceous pitches are well known as useful in the formation of a wide variety of carbon artifacts. One such artifact of particular commercial interest today is carbon fiber. For convenience, particular reference will be made herein to carbon fiber technology, but it will be appreciated that the present invention has applicability in areas other than carbon fiber formation.
Carbon fibers are used in reinforcing plastic and metal matrices where the exceptional properties of the reinforcing composite materials such as their high strength to weight ratios clearly offset the general higher cost associated with their preparation. It is generally accepted that large scale use of carbon fibers as a reinforcing material would gain even greater acceptance in the marketplace if the costs associated with the formation of such fibers could be substantially reduced. As a result, the formation of carbon fibers from relatively inexpensive carbonaceous pitches has received considerable attention in recent years.
High strength, high modulus carbon fibers prepared from pitches are characterized in part by the presence of carbon crystallites which are preferentially aligned parallel to the fiber axis. The highly oriented structure of the carbon fibers has been obtained either by introducing orientation into the carbon fiber by high temperature stretching or by first forming a pitch fiber which possesses considerable anisotropy.
In forming carbon fiber from the pitch material with a high degree of orientation, it has generally been considered necessary to thermally transform the carbonaceous pitch prior to fiber formation, at least in part, to a liquid crystal or so-called mesophase state. Typically, the thermal transformation is achieved at temperatures between about 350.degree. and about 500.degree. C. for exceedingly long periods of time. For example, at the minimum temperature generally required to convert an isotropic pitch to the mesophase state of 350.degree. C., at least one week of heating is usually necessary and the mesophase content of the pitch is only about 40% with the balance being isotropic material. At higher temperatures of, e.g., about 400.degree. C., at least 10 hours of heating are usually necessary.
A wide variety of complex reaction sequences take place during the thermal treatment of isotropic pitches and these reactions result in the formation of large parallel aligned lamellar optically anisotropic molecules which are known as mesophase pitch. Small insoluble liquid spheres begin to appear in the pitch and gradually increase in size as the heating is continued. Ultimately the spheres begin to coalesce into large domains that display strong optical anisotropy, which is characteristic of parallel alignment of the liquid crystal phase. This mesophase transformation has been followed quantitatively by polarized light microscopy investigations of solvent extracted samples in which the untransformed isotropic matrix is dissolved in a solvent such as pyridine or quinoline and the insoluble mesophase fraction is recovered by filtration.
More recently, it was discovered that isotropic carbonaceous pitches contain a separable fraction which is capable of being converted very rapidly, indeed generally in less than about 10 minutes and especially in less than 1 minute when heated to temperatures in the range of about 230.degree.-400.degree. C., to a strong optically anisotropic deformable pitch containing greater than 75% of a liquid crystal type structure. The highly oriented anisotropic pitch material formed from only a fraction of an isotropic carbonaceous pitch has substantial solubility in pyridine and quinoline and consequently, such material is referred to as a neomesophase pitch. This process is described in U.S. Pat. No. 4,208,267.
The neomesophase fraction of pitch is isolated by solvent extraction of well known commercially available graphitizable pitches such as Ashland 240 and Ashland 260. The amount of neomesophase fraction of the pitch that is separable, however, is relatively low. For example, with Ashland 240, no more than about 10% of the pitch constitutes a separable fraction capable of being thermally converted to neomesophase. It was discovered that isotropic carbonaceous pitches could be pretreated in such a manner as to increase the amount of that fraction of the pitch which is separable and capable of being converted very rapidly to a deformable pitch containing greater than 75%, and especially greater than 90%, of a liquid crystal type structure. The pretreatment involves heating a typical graphitizable, isotropic carbonaceous pitch at an elevated temperature for a time sufficient to increase the amount of that fraction of the pitch that is capable of being converted to neomesophase and terminating such heating at a point in time when spherules visible under polarized light appear in the pitch, and preferably at a point which is just prior to the formation of the visible spherules. The pretreatment is described in detail in U.S. Pat. No. 4,184,942.
The known processes for producing the neomesophase fraction involve the step of treating a carbonaceous isotropic pitch with an organic solvent system which is characterized by having a solubility parameter at 25.degree. C. of between about 8.0 and about 9.5. The solubility parameter has preferably been 8.7-9.2. The solubility parameter, .delta., of a solvent or a mixture of solvents is given by the expression: ##EQU1## wherein H.sub.v is the heat of vaporization of the material, R is the molecular gas content, T is the temperature in degrees Kelvin and V is the molar volume. In this regard, see, for example, J. Hildebrand and R. Scott, "Solubility of Non-Electrolytes", 3rd Ed., Reinhold Publishing Company, New York (1949) and "Regular Solution", Prentice Hall, New Jersey (1962). Typical solubility parameters at 25.degree. C. are 9.0 for benzene, 8.7 for xylene and 8.2 for cyclohexane. Heretofore, the preferred solvent has been toluene which has a solubility parameter of 8.8.
It has now been surprisingly discovered that certain solvents whose solubility parameter is greater than 9.5 or solvent systems based thereon can be used in place of the organic solvent systems having a solubility parameter of between about 8.0-9.5.
Accordingly, it is the object of this invention to provide a process for producing an optically anisotropic pitch with an organic solvent system whose solubility parameter is greater than 9.5. This and other objects of the invention will become apparent to those skilled in the art from the following detailed description.